lib/crc: Prepare for arch-optimized code in subdirs of lib/crc/

lib/crc: Prepare for arch-optimized code in subdirs of lib/crc/

Rework how lib/crc/ supports arch-optimized code.  First, instead of the
arch-optimized CRC code being in arch/$(SRCARCH)/lib/, it will now be in
lib/crc/$(SRCARCH)/.  Second, the API functions (e.g. crc32c()),
arch-optimized functions (e.g. crc32c_arch()), and generic functions
(e.g. crc32c_base()) will now be part of a single module for each CRC
type, allowing better inlining and dead code elimination.  The second
change is made possible by the first.

As an example, consider CONFIG_CRC32=m on x86.  We'll now have just
crc32.ko instead of both crc32-x86.ko and crc32.ko.  The two modules
were already coupled together and always both got loaded together via
direct symbol dependency, so the separation provided no benefit.

Note: later I'd like to apply the same design to lib/crypto/ too, where
often the API functions are out-of-line so this will work even better.
In those cases, for each algorithm we currently have 3 modules all
coupled together, e.g. libsha256.ko, libsha256-generic.ko, and
sha256-x86.ko.  We should have just one, inline things properly, and
rely on the compiler's dead code elimination to decide the inclusion of
the generic code instead of manually setting it via kconfig.

Having arch-specific code outside arch/ was somewhat controversial when
Zinc proposed it back in 2018.  But I don't think the concerns are
warranted.  It's better from a technical perspective, as it enables the
improvements mentioned above.  This model is already successfully used
in other places in the kernel such as lib/raid6/.  The community of each
architecture still remains free to work on the code, even if it's not in
arch/.  At the time there was also a desire to put the library code in
the same files as the old-school crypto API, but that was a mistake; now
that the library is separate, that's no longer a constraint either.

Reviewed-by: "Martin K. Petersen" <martin.petersen@oracle.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Acked-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
Link: https://lore.kernel.org/r/20250607200454.73587-3-ebiggers@kernel.org
Link: https://lore.kernel.org/r/20250612054514.142728-1-ebiggers@kernel.org
Link: https://lore.kernel.org/r/20250621012221.4351-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@kernel.org>